For a given airspeed, an aircraft may consume less fuel at a higher altitude than it does at a lower altitude. In other words, an aircraft may be more efficient in flight at higher altitudes as compared to lower altitudes. Moreover, bad weather and turbulence can sometimes be avoided by flying above such weather or turbulence. Thus, because of these and other potential advantages, many aircraft are designed to fly at relatively high altitudes.
As the altitude of an aircraft increases, from its take-off altitude to its “top of climb” or “cruise” altitude, the ambient atmospheric pressure outside of the aircraft decreases. Thus, unless otherwise controlled, air could leak out of the aircraft cabin causing it to decompress to an undesirably low pressure at high altitudes. If the pressure in the aircraft cabin is too low, the aircraft passengers may suffer hypoxia, which is a deficiency of oxygen concentration in human tissue. The response to hypoxia may vary from person to person, but its effects generally include drowsiness, mental fatigue, headache, nausea, euphoria, and diminished mental capacity.
Aircraft cabin pressure is often referred to in terms of “cabin altitude,” which refers to the normal atmospheric pressure existing at a certain altitude. Studies have shown that the symptoms of hypoxia may become noticeable when the cabin altitude is above the equivalent of the atmospheric pressure one would experience outside at 8,000 feet. Thus, many aircraft are equipped with a cabin pressure control system to, among other things, maintain the cabin pressure altitude to within a relatively comfortable range (e.g., at or below approximately 8,000 feet) and allow gradual changes in the cabin altitude to minimize passenger discomfort.
A typical cabin pressure control system implements pressure controllers, outflow valves and control logic that may, when needed or desired, begin pressurizing the aircraft cabin (or “descending” the aircraft cabin) before take-off, either while taxiing on or to the runway or at the start of the take-off roll down the runway. The pressure control system may implement pressure transducers, also referred to as pressure sensors, to monitor various pressure conditions internal and external the aircraft cabin. During flight the pressure control system enables the aircraft cabin to remain at the appropriate pressure until landing, when de-pressurization takes place. Typically, the pressure sensor components of the cabin pressure system are initially calibrated when placed into service to primary standards set forth by the National Institute of Standards and Technology (NIST). Oftentimes these sensor components are subject to drift due to age, external environmental factors or influence of other system components. To compensate for the drift, the pressure control system hardware or operational software may be fine tuned at various times during the life of the system. When the drift exhibited by the pressure sensors becomes excessive, the sensor components are typically removed from the aircraft and recalibrated, resulting in down time, expense and loss of revenue.
Hence, there is a need for a method of auto-calibrating a pressure control system, and more particularly the system's pressure sensors, without the need for removal of the pressure sensor components from the aircraft. More particularly, there is a need for a method that enables the drift experienced by the pressure sensor components to be auto-calibrated while the components are in service. The present invention addresses one or more of these needs.